EFO is shorthand for "Error, Freak, or Oddity". It's a term applied to philatelic items that are not normal. The degree of "not normal" can mean substantial sums of money sometimes. How do you decide what's an error versus a freak or oddity?
Errors usually have catalog status. Something has gone completely wrong. Examples are missing perforations (imperf between copies or completely imperf copies), missing color errors, missing tagging, and so forth. Many collectors are familiar with Scott #C3a, the inverted Jenny and Scott #1610c, the inverted candle holder or "CIA Invert" as it's sometimes referred to.
Some errors are very common. For example, Scott #1895d is the imperf pair of the 20¢ Flag stamp. It has a catalog value of $8.50 and is very common. At the other extreme is Scott #C3a with a catalog value of $500,000.
Freaks are stamps with a usually more visually striking appearance. They do not have catalog status. Examples of freaks include major perforation shifts, partially missing colors, pre-printing paper folds, over or under inked stamps, and so forth. Freaks often sell in the range of a few dollars. Some really spectacular appearing freaks will bring in the tens or a few hundreds of dollars.
Oddities is the catch-all category for anything that is left. Oddities are often very subtle problems and do not have catalog status. Examples of oddities are inverted date slugs on USPS cancellations, very minor perforation shifts, minor color shifts, and so forth. Most oddities have very little retail value and are generally more in the "curiosity" category.
Errors are very clear cut. What's the difference between a freak and oddity though? Often times, it's a matter of personal opinion. Take pre-printing paper folds for instance. This is when the paper develops a fold before the stamp is printed. When the fold is opened up, there is a gap in the stamp design. Most paper folds are very trivial and are less than 1 mm. wide and generally considered oddities. Some paper folds are quite large, several millimeters across. They are considered freaks due to their unusually large size. Where is the dividing line between oddity and freak? Is it 2 mm? Perhaps 3 mm? Maybe 4 mm? What may be an oddity to one collector may be a freak to someone else.
What makes EFOs so interesting? Part of it is eye appeal. The errors and freaks are very unusual looking. But the most important consequence of collecting EFOs is that it teaches you stamp production methods. A collector will see an EFO and say, "How did that happen?" That often leads one into digging into the Scott catalog or some other philatelic reference work to find out how those stamps were produced and how that EFO occurred. Many EFOs are easy to identify exactly what went wrong and some items are the subject of speculation.
If you are interested in learning more about EFOs, I encourage you to look into the Errors, Freaks, and Oddities Collectors Club (EFOCC). If you have access to the Internet, you can check out the club website at www.efocc.org. Or contact the club secretary: Mr. Stan Raugh, 4217 8th Ave., Temple, PA 19560-1805. I've been an EFOCC member for years and I would be happy to send you an application too. The EFOCC is a great club!